The programming terminal provides access to the FORCE instruction, which is a fairly standard instruction on most PLC's. By using the FORCE key, it is possible to test the input and output status of any device interfaced at the I/O rack.

For example, forcing a solenoid vavle ON will immediately tell you wether the solenoid is functional when the program is bypassed. If it is, the problem must be related to the software, and not the hardware. If the output fails to respond when forced, it is generally either the actual output module causing the problem, or the solenoid itself is malfunctioning.

Since FORCE commands must be executed when the system is fully powerd, it is necessary to use extreme caution when performing this type of test. However, the FORCE instruction is a very useful troubleshooting tool and is highly recommended for locating problems related to a PLC.